The use of screws in orthopedic surgery extends back to the latter part of the 1800's, with Riguad implanting Swedish steel screws for repairing a fracture of the olecranon. By 1866, Hansmann of Hamburg, Germany developed the first bone plate and screw device assembly, with the screws being inserted pericutaneously. By the early 1900's, the problem of the screws loosening in the bone socket in which they were inserted was recognized as a significant problem. In response to this problem, new screw designs were developed. Lane designed a screw from wood. However, poor hold in diaphyseal bone led other to use metal for the screw design. Self-tapping screws for orthopedics were developed as well around 1921.
Presently, there are numerous screw designs using different threads and materials. Single and multiple lead threaded screws, with or without self-tapping capabilities, can be found throughout all orthopedics. The most common materials used for these screws are titanium, cobalt chrome, and stainless steel. Bioresorbable screws are also used, made from various compositions well known in the art. Examples of bioresorbable materials commonly used in today's orthopedics include polylactic acid, polyglycolic acid, the L-Isotope form of polylactic acid, and copolymers of polyactic acid and polyglycolic acid.
In spite of recent developments, the basis of the bone screw has remained unchanged, even though loosening of the screw in the socket into which it is seated has become more of an issue with more recent applications of technology. To compensate for these problems, various approaches, such as coatings and better bone inductive or conductive materials have been applied to the surface of the screw. Altering the screw by texturing the screw surface has also been attempted. While these approaches may in some ways address the problem, they are not sufficient to address the current problem of loosening with a number of high-load applications.
A significant part of the correct screw insertion and fixation in any application is the necessity of sufficient “bite” or depth of the actual thread into the bone. In practice, this is accomplished ideally by having an entry hole for the screw matching a minor diameter, such that the entire thread extending from the minor diameter to the major diameter is completely buried in the bone itself. It is apparent that too small a screw will have insufficient thread purchase and be subject to being pulled out of the bone. Too large a screw relative to the entry-hole size creates the risk of overstressing the bone and causing fractures. If sized correctly, a screw will give good holding values, or what is termed pull-out strength, initially. However, what happens under high loads or bone remodeling is a loosening of the screw within its socket.
A number of more recent spinal systems have moved toward the concept of dynamic systems. In these systems, the screw remains the anchor, but the loads are distributed or altered by a device placed between the screws. One such system uses a woven cord to allow flexure in certain directions, but rigidity where needed. Other systems utilize polyetheretherketone (PEEK) polymer rods, flexible rods, or flexible connectors. One aspect that all these have in common is that they change the load on the screw fixation means significantly. Bone screws experience higher loads and flexion/extension of the spine places cyclic loads on the screws which differ than the previous more rigid rod fixation means. This often results in much higher loosening rates in vivo. One current system reports loosening failure rates anywhere from 8%-39%. As expected, the numbers vary with the number of patients, but regardless, 8% failure rate is not an acceptable level, let alone a 39% failure.
The root of the problem discussed above lies in the means of fixation to the bone. Regardless of how the surface of the screw is treated, the technology in screw means of fixation remains almost the same as screws developed in the early 1920's.
Other orthopedic devices face similar drawbacks with regard to bone gripping and remodeling. For example, it is desirable to stabilize cervical interbody fusion systems, such as cages. Such cages often depend on bone healing from one vertebrae, through a cage, to another vertebrae.
In order to address and resolve the problem of loosening, it is a far better approach to allow the screw to adjust to bone remodeling or bone interface damage. Bone interface damage, such as a screw thread being pulled out of the thread in the bone, effectively strips the threads. In a normal screw, loosening then occurs.